1. Field of the invention
The present invention relates to a superconducting device and a method for manufacturing the same, and more specifically to a superconducting device having an extremely thin superconducting channel formed of oxide superconductor material, and a method for manufacturing the same.
2. Description of related art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconducting material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature.
Josephson device is one of well-known superconducting devices. However, since Josephson device is a two-terminal device, a logic gate which utilizes Josephson devices becomes complicated configuration. Therefore, three-terminal superconducting devices are more practical.
Typical three-terminal superconducting devices include two types of super-FET (field effect transistor). The first type of the super-FET includes a semiconductor channel, and a superconductor source electrode and a superconductor drain electrode which are formed closely to each other on both side of the semiconductor channel. A portion of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode has a greatly recessed or undercut rear surface so as to have a reduced thickness. In addition, a gate electrode is formed through a gate insulating layer on the portion of the recessed or undercut rear surface of the semiconductor layer between the superconductor source electrode and the superconductor drain electrode.
A superconducting current flows through the semiconductor layer (channel) between the superconductor source electrode and the superconductor drain electrode due to the superconducting proximity effect, and is controlled by an applied gate voltage. This type of the super-FET operates at a higher speed with a lower power consumption.
The second type of the super-FET includes a channel of a superconductor formed between a source electrode and a drain electrode, so that a current flowing through the superconducting channel is controlled by a voltage applied to a gate formed above the superconducting channel.
Both of the super-FETs mentioned above are voltage controlled devices which are capable of isolating output signals from input ones and of having a well defined gain.
However, since the first type of the super-FET utilizes the superconducting proximity effect, the superconductor source electrode and the superconductor drain electrode have to be positioned within a distance of a few times the coherence length of the superconductor materials of the superconductor source electrode and the superconductor drain electrode. In particular, since an oxide superconductor has a short coherence length, a distance between the superconductor source electrode and the superconductor drain electrode has to be made less than about a few ten nanometers, if the superconductor source electrode and the superconductor drain electrode are formed of the oxide superconductor material. However, it is very difficult to conduct a fine processing such as a fine pattern etching, so as to satisfy the very short separation distance mentioned above.
On the other hand, the super-FET having the superconducting channel has a large current capability, and the fine processing which is required to product the first type of the super-FET is not needed to product this type of super-FET.
In order to obtain a complete ON/OFF operation by a signal voltage of several volts applied to the gate electrode, both of the superconducting channel and the gate insulating layer should have an extremely thin thickness. For example, the superconducting channel formed of an oxide superconductor material should have a thickness of less than five nanometers and the gate insulating layer should have a thickness more than ten nanometers which is sufficient to prevent a tunnel current.
The super-FET has a superconducting source region and a superconducting drain region which have a sufficient thickness for forming contacts on them, for example more than 200 nanometers. Since, the relatively thick superconducting source region and the superconducting drain region are arranged at the both ends of the extremely thin superconducting channel, the super-FET may not have a planar upper surface. In this case, resolution of photolithography is spoiled so that the super-FET can not be manufactured so as to have a required fine structure. The amount of the integration of the super-FET is also limited by the resolution of photolithography.
In addition, in a prior art, in order to increase the superconducting current through interfaces between the superconducting channel and the superconducting source region and between the superconducting channel and the superconducting drain region, the superconducting source region and the superconducting drain region may be processed by etching so that they are smoothly connected to the superconducting channel.
However, the oxide superconductor thin films which constituted of the superconducting source region and the superconducting drain region are degraded during the etching so that the superconducting characteristics is affected. In addition, the etched surface of the oxide superconductor thin film is roughened. Therefore, if an oxide superconductor thin film which is constituted of the extremely thin superconducting channel is formed on the rough surface, its crystallinity and the superconducting properties are also affected. Additionally, an undesirable Josephson junction or resistance is generated at these interfaces.
By this, the super-FET manufactured by the above conventional process does not have an enough performance.